Radial refractive optical gradients have been generated in samples of plastic and glass. In U.S. Pat. No. 3,718,383 for "Plastic Optical Element Having Refractive Index Gradient", issued to Robert S. Moore on Feb. 27, 1973, the inventor describes the diffusion of a diluent into a shaped polymeric matrix to form a continuous gradient in refractive index in a direction perpendicular to the optical axis thereof. The diluent and the polymeric material have different refractive indices. In cylindrical samples, an angularly symmetric, radial gradient of refractive index substantially proportional to the radial distance perpendicular to the optical axis may be formed by diffusion of a diluent having lower index of refraction than the plastic matrix material into the matrix from the central core thereof. Similarly, for positive lenses, where the refractive index must decrease in the outward radial direction, inward diffusion of a diluent external to a plastic rod is required.
In U.S. Pat. No. 3,859,103 for "Optical Glass Body Having A Refractive Index Gradient", issued to Mitsugi Yoshiyagawa on Jan. 7, 1975, the inventor describes the production of a continuously decreasing index of refraction from the central axis of a glass object to the peripheral surface thereof as a result of the substitution of thallium ions contained in the glass by external alkali metal ions. Glass containing Tl.sub.2 O was chosen since the thallium ions give the glass a high refractive index. The process for achieving the required substitution of ions is to bring the glass article into contact with a chosen molten salt for a period of time sufficient for significant diffusion to take place. A distribution of the refractive indices according to the relationship N=N.sub.O (1-ar.sup.2) was generated in a glass rod, where r is the distance from the center in the radial direction, a is a positive constant, and N.sub.O is the refractive index at the center of a cross section of the glass body perpendicular to the central axis thereof.
In U.S. Pat. No. 4,053,204, "Optical Fiber Having Reduced Dispersion", issued to Stewart E. Miller on Oct. 11, 1977, and in U.S. Pat. No. 4,076,380, "Graded Index Optical Fiber", issued to Frank Vincent DiMarcello and John Charles Williams on Feb. 28, 1978, the inventors disclose graded refractive index optical fibers having radial gradients in repetitively varying discrete longitudinal zones for improving the dispersion characteristics of light pulses traveling therethrough. In the former patent, the zones are achieved by varying the thickness of each layer of constant optical index material, while in the latter patent, layers of different index of refraction are disposed in a helical pattern along the length of the fiber. Chemical vapor deposition techniques are used to form the layers in both devices.
In U.S. Pat. No. 4,696,552, "Projection Device with Refractive Index Distribution Type Lens", issued to Jun Hattori and Shigeyuki Suda on Sept. 29, 1987, the inventors disclose a projection device having an illuminating system for illuminating an object, and an index distribution type lens for projecting the image of the object. The lens has a refractive index distribution substantially proportional to the square of the distance from the optic axis in a cross-section perpendicular to the optic axis and a refractive index distribution monotonously varying in the direction of the optic axis. The lens is characterized by dimensions of about 18 mm in length and 0.5 mm in diameter (perpendicular to the optic axis) and a change in refractive index of less than 0.05.
For the purpose of the present specification, we define the term "optical axis" to mean an imaginary straight line which extends internally through the refractive material of the subject invention and which passes through both the entrance and exit surfaces of this material which are adapted for the passage of light. Although there may be more than one optical axis for a chosen embodiment of the invention, in general, the optical axis will be uniquely defined by the geometrical symmetry of the material. In either event, changes in the index of refraction of the refractive material will be defined relative to the optical axis. Also for the purpose of the present specification, we define the term "bidirectional gradient" to refer to a gradient in the index of refraction that occurs along each of two directions, usually mutually orthogonal. Finally, "light" is defined as that electromagnetic radiation in the frequency spectrum ranging from infra-red through visible to ultraviolet.
Notably absent from the patent literature and from the science and engineering literature is a description of monotonically varying distributions of optical densities with significant change in index of refraction and over significant dimension in the axial direction. While Hattori et al, supra, disclose lenses having bi-directional gradient varying indices of refraction, such lenses have no substantial thickness, as the term is used herein, and no significant change in index of refraction, as the term is used herein.
Recent advances in fabricating macro-gradient optical density transmissive light concentrators, lenses and compound lenses of large geometry have been disclosed in Ser. No. 07/206,109 now Pat. No. 4,883,522 and 07/206,110, both filed June 17, 1988, and assigned to the same assignee as the present application. These applications disclose the use of glass powders, or frits, to fabricate the optical elements; a series of frit mixtures, ranging from a first composition to a second composition in small steps, e.g., 10% change per layer of frit, are placed in a crucible and melted.
The optical elements derived from the frit process are suitable for the purposes intended. Nevertheless, advances in processing techniques are required in order to provide improved optical elements.